The central goal of our PROGRAM is to attempt to selectively tolerize only the small numbers of allospecific T cells transferred in the donor marrow (BM) that are responsible for GVHD. By leaving greater than 99 percent of donor T cells adoptively transferred in the BM functionally intact, we hope to minimize graft failure yet retain immunity to pathogens and tumor cells. Moreover, we hope that this approach will permit us to decrease non-specific immunosuppression that decreases the host's capacity to respond to opportunistic infections. To achieve these objectives, we plan to specifically ex vivo anergize alloreactive T cells in the donor BM to host alloantigen. We have shown that blockade of B7 family mediated costimulation is necessary to induce alloantigen specific T cell clonal anergy and that the frequency of donor host specific alloractive precursor helper T cells (pHTL) can be reduced to below that thought to be associated with a significant incidence of GVHD. We are in the clinic with this methodology and preliminary evidence shows that the donor host pHTL frequency can be reduced to levels below that predictive for GVHD. The first project will develop and evaluate in clinical experimentation therapeutic modalities to inhibit allorecognition specifically while leaving intact the remaining immune repertoire. This may allow us to decrease non-specific toxicity while preserving or improving upon current standards of GVHD control. In the second project, using our human T cell clonal system, we plan to define precisely which additional molecules might prevent the induction of anergy, investigate whether CD8plus T cells can be anergized, and finally, to continue our efforts to decipher the biochemical basis for the anergic defect. The primary goal of the third project will be to study the in vivo requirements for costimulation in naive and memory T cell responses to alloantigen. These studies should provide new insights that are highly relevant to the consideration of the GVHD risk of cord blood versus adult BM hematopoietic sources. This Program has been highly interactive since its genesis. To ensure its success, we have assembled a highly diverse yet interactive collaborative team of molecular biologist, immunologists, transplant biologists, and clinicians with a long track record of successful collaboration and with extensive translational experience to drive our basic science discoveries to clinical experimentation.